The present invention generally relates to processes, systems, and equipment capable of removing gases and particulate matter and gases from flue gases. The invention particularly relates to wet flue gas desulfurization (FGD) processes, systems, and equipment with which potassium sulfate can be produced as a byproduct of sulfur dioxide removal from flue gases using an ammonia-containing solution.
Gas-liquid contactors and absorbers are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, and their emission into the atmosphere is regulated by clean air statutes. Methods by which these gases are removed with gas-liquid contactors and absorbers have been referred to as wet flue gas desulfurization.
The cleansing action produced by a gas-liquid contactor is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that cleans the gas. Wet flue gas desulfurization processes have typically involved the use of calcium-based slurries or sodium-based or ammonia-based solutions. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO3) slurries and hydrated lime (calcium hydroxide; Ca(OH)2) slurries formed by action of water on lime (calcium oxide; CaO). Such alkaline slurries react with the acidic gases to form precipitates that can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases that are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry and the formation of salts such as, in the case of calcium-based slurries, calcium sulfite (CaSO3.½H2O), gypsum (CaSO4.2H2O), calcium chloride (CaCl2), and calcium fluoride (CaF2). Forced oxidation of the slurry by aeration is often employed to ensure that all of the sulfites will be reacted to form sulfates, which in the case of a calcium-based slurry serves to maximize the production of gypsum.
While gas-liquid contactors and absorbers utilizing calcium-based slurries as described above generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter often having only nominal commercial value. In contrast, ammonia-based scrubbing processes produce a more valuable ammonium sulfate fertilizer. In these processes, sulfur dioxide within the flue gas reacts with ammonia (NH3) to form an ammonium sulfate solution or ammonium sulfate crystals ((NH4)2SO4). A particular example of such a process is disclosed in U.S. Pat. No. 5,362,458 and results in the production of ammonium sulfate fertilizer by reacting sulfur dioxide and free ammonia (NH3) in an ammonia-containing scrubbing solution. In certain markets, the added value of ammonium sulfate over the value of ammonia is minimal. In addition, some prior art processes have required bulk supplies of ammonia that are consumed by the desulfurization process, necessitating the transportation and on-site storage of large quantities of ammonia. Because transportation and storage of ammonia are highly regulated and relatively costly, under certain circumstances the production of ammonium sulfate using flue gas desulfurization systems has been viewed by some in the industry as better suited for use in niche markets.
U.S. Pat. No. 5,624,649 discloses a process capable of enhancing economic aspects of desulfurization processes by producing a byproduct having of greater market value than ammonium sulfate. In particular, U.S. Pat. No. 5,624,649 discloses reacting flue gases with ammonia to form an ammonium sulfate solution, and then reacting the ammonium sulfate solution with potassium chloride (KCl) to produce potassium sulfate (K2SO4) in a manner than is capable of achieving a high yield of both potassium and sulfate. While the process is very effective for its intended purpose, the resulting potassium sulfate crystals may be small (for example, an average major dimension of 0.2 mm or less) and therefore somewhat difficult to filter and subsequently handle. In addition, certain steps of the process involve handling a solution, slurry, or other material that may contain a high concentration of free ammonia (NH3), which can lead to higher operating costs in order to contain the ammonia and/or may, under some circumstances, result in ammonia losses. Also, the potassium chloride salt is dissolved at ambient temperature to maintain the free ammonia in solution, resulting in a relatively slow dissolution rate that may be offset in part with the use of a relatively large and expensive reaction vessel.
From the above, it would be desirable if further advances in flue gas desulfurization processes were available to produce potassium sulfate as a valuable byproduct.